Friday 6 November 2009 12.30 EST
First published on Friday 6 November 2009 12.30 EST

The investigation into Sir Mark Thatcher's role in the failed coup in Equatorial Guinea could be hindered by legal obstacles, which may mean he could never be prosecuted in this country, according to senior law enforcement sources.

Scotland Yard detectives are engaged in what is said to be a serious and protracted inquiry into allegations that Old Etonian Simon Mann, along with Thatcher and others, orchestrated the attempt to oust the country's president during meetings in London in late 2003 and early 2004.

According to the former prosecutor in Equatorial Guinea, detectives from the counter-terrorism command at the Yard visited Mann in prison five times. He is said to have co-operated fully.

The evidence relating to Thatcher's alleged involvement in the coup as a key player is believed to be held in testimonies given to Yard officers by the British mercenary, and in documents handed to the Metropolitan police by Equatorial Guinea and held by officials in South Africa.

But despite months of gathering evidence in a detailed inquiry, investigators could stumble when it comes to considering any charges against the son of the former prime minister.

As Thatcher has already been convicted in South Africa of paying for a helicopter he suspected might be used for mercenary activity, the likelihood that he could be prosecuted in this country on the same evidence for a similar offence is very slim, according to senior legal and police sources. When he talks to the police in the UK, Mann will have to come up with something new and powerful if the prosecution is to go ahead, it is understood.

The sources indicated that the pursuit of Thatcher was primarily hindered by fears of double jeopardy because of his South African conviction, despite the fact that this took place in a different jurisdiction.

Adrian Chaplin, a criminal barrister in London, agreed. He said there were potentially huge problems of abuse of process should Thatcher be charged with anything.

"If there was an attempt to take him through the courts here there would be a fairly powerful argument to say: 'What's the point because he has been convicted and punished already,'" he said.

Thatcher was first implicated in the coup attempt in a letter written by Mann and intercepted by the South African police while he was being held in Harare, after his arrest following the failed plot in 2004. Written to his wife in Cape Town it made a request for money from the alleged financiers Scratcher, his nickname for Thatcher, and Smelly, a nickname for the Lebanese-born businessman Ely Calil.

Both Thatcher and Calil deny involvement in the coup attempt.

Other evidence which could be used against Thatcher is the testimony of Crause Steyl, a South African mercenary pilot who was on the plane the night of the coup. He was never arrested and returned to South Africa where a deal was struck. He co-operated with the authorities by telling them everything he knew, and he talked about Thatcher. According to Adam Roberts, author of The Wonga Coup, a detailed account of the plot, that was when the whole story about Thatcher exploded. Rumours about his involvement had been swirling for some time, but with Steyl's plea bargain evidence the South Africans arrested Thatcher in a move watched with disbelief by the world's press.

In South Africa Thatcher eventually admitted breaking the country's anti-mercenary laws, by giving money to someone despite suspecting the cash would be used for mercenary activity. Initially Thatcher told the authorities he had thought the £288,000 was to be used to fund an air ambulance for Africa's poor. But he accepted that at some point before the coup attempt, he suspected the helicopter might be used for a mercenary plot.

Thatcher was convicted in January 2005, fined £266,000 and given a four-year suspended sentence. He left South Africa shortly afterwards.

The third plank of evidence which could be used against him is Mann's testimony during his interviews with Scotland Yard detectives, who visited him in Black Beach prison after his trial ended in Equatorial Guinea last July. Mann implicated Thatcher, as he had done at his trial, when he said Thatcher was "not just an investor. He came on board completely and became part of the management team."

He also is understood to have continued to implicate Calil, who he named at his trial, saying he was the "overall boss".

But Mann's testimony will be one of the legal problems. The statements he made in Equatorial Guinea are tainted by the threat of ill-treatment or even death in prison. He is likely to repeat his statements under caution to the British police but even away from the threat of torture or death, his testimony might not be enough to lead to a prosecution of Thatcher.

Equatorial Guinea is still seeking to see Thatcher in court. President Teodoro Obiang said Thatcher would "definitely" be brought to justice. But his government has not asked Spanish police to arrest him.